1. Field of the Invention
This invention relates to methods for sealing heat sealable film wrapped around a series of products, and more particularly to sealing bi-axially oriented film wrapped around a product.
2. Description of the Related Art
In the preparation of various products for market presentation to the consumer, many are wrapped in a clear film to protect the product from the environment and yet permit product visibility. Common among the products which are so wrapped are food items, especially poultry parts. The packaging of poultry parts for sale is frequently done with a film which can be shrunk when heated. The shrinking helps to achieve a smooth and attractive appearance of the package by pulling the film tightly around the product.
In the process of wrapping products such as those which are the subject of the invention, a sheet of heat sealable film of indefinite length is formed into an elongate tubular configuration, in some instances with the aid of a "former", as disclosed in U.S. Pat. No. 4,574,566 to Eaves et al., the teachings of which are incorporated herein by reference. In this process, for example, a series of products are wrapped into a film tube in linear alignment generally along the axis of the tube and are longitudinally (used herein to be in the direction of travel) spaced apart from one another. A longitudinal seam is formed by joining the opposed edges of the film sheet, and a transverse (used herein to be normal to the direction of travel) seam is formed between each of successive leading and trailing packages by means of heat and pressure. As each transverse seam is sealed, the heating substantially melts the film so that the leading package separates from the trailing package. The '566 patent also discloses a sealing mechanism for forming a transverse seam in the film tube between sequential products in which a pair of opposed rotary sealing heads are caused to revolve so as to match the direction and linear speed of the product and tube as portions of the sealing heads press upper and lower surfaces of the tube together and apply heat. According to the teaching of the '566 patent, several portions of the packaging machine conveyor are each driven by a different servomotor so as to be able to synchronize speeds more precisely than was considered possible by a mechanical drive system. However, no teaching or suggestion of speed variation between conveyor portions is made.
Another arrangement for the operation of sealing bars for producing transverse seams is disclosed in U.S. Pat. No. 5,271,210 to Tolson, the teachings of which are also incorporated herein by reference. In the apparatus described in the '210 patent, a pair of opposed linear sealing bars are moved vertically and are brought into contact with the upper and lower surfaces of the film tube as the sealing bars are conveyed in the direction of travel of the products and the tube. This action results in the contact time between the sealing bars and the film tube being maximized without slowing the speed of product conveyance. The sealing bars are then separated vertically and conveyed in the opposite direction back to their starting point. Depending on the package being wrapped and film being used, this equipment can be operated as fast as 70 packages per minute.
The '210 patent also discloses a pair of vertically oriented lateral control belts associated with the sealing bars and which are mounted and driven so as to be able to move linearly in a direction opposite to that in which the products are moved while the belts rotate in the direction of the product movement and move linearly in the same direction as the products move when the sealing bars are separated and are returning to their upstream position and while the next product is moved past the sealing bars. The lateral control belts move linearly in synchronization with the products without belt rotation while the sealing bars are pressed together to seal the transverse seam.
Sealing of two portions of heat sealable film involves heating one or both portions to near the melting point and pressing the two portions together. A common problem encountered due to the heat applied is that of film shrinkage in either or both the longitudinal and transverse directions of the film. This problem has been dealt with in various ways as next described.
Known apparatus for forming a transverse seam between successive packages includes clamps with circulating cooling fluid. A first known and typical sealing apparatus uses a cooled clamp in conjunction with an electrical resistance heating wire in a manner to instantaneously heat and deactivate the wire while continuing to hold the film with the cooling clamps. This generally results in a transverse seam being formed without allowing longitudinal shrinkage in the film.
A second known sealing apparatus utilizes a pair of clamping jaws flanking a heat sealing element upstream and a second such apparatus downstream of a film-parting knife. This apparatus prevents the film from shrinking in both the longitudinal and the transverse directions.
A third known heat sealing apparatus employs spring-mounted cooled clamps and a rigidly mounted sealing bar which are all mounted to a common carrier bar. This apparatus is discussed in detail below as the preferred apparatus with which to practice the method of the invention. This third type heat sealing apparatus has been used in a Model 500E packaging machine, sold by Ossid Corporation of Rocky Mount, N.C.
A particularly effective film for use in shrink-wrapping is a film which prior to being used as a wrapping film has been oriented, or work stretched, in two perpendicular directions. This type of film, known as bi-axially oriented film, when subjected to heat, shrinks bi-axially, thus removing wrinkles in both directions. Such a bi-axially oriented film is supplied by the Cryovac Division of W. R. Grace Chemical Co., Inc. as style SSD-310. The degree of shrink is proportional to the degree of orientation and of the heat applied. If a film is wrapped around a product, it is typically constrained and is not able to shrink to the extent it would if left unconstrained. Consequently, a residual component of film tension is created in the film. Subsequent movement of the product provokes film contraction in response to this film tension and during heat sealing in the manner described above may cause the formation of non-uniform and relatively weak seam.
Typically, poultry products are handled and wrapped while frozen or semi-frozen (firm) at a temperature of about -15.degree. C. (0.degree. C.). These same poultry products are displayed in a retail store at a temperature sufficient to maintain freshness and enhance appearance, i.e. 1.degree. C. (34.degree. F.). Thus, both the sealing system and the film employed must accommodate to a wide range of temperatures.
The longitudinal seam formed along the bottom of the package is considered to be generally uniform and satisfactory. However, according to all known heat sealing apparatus and methods, seal integrity of the transverse seam is marginal and fairly irregular in form.
It is therefore an object of this invention to provide an improved sealing method capable of controlling the movement of a heat shrinkable film during heat sealing to establish a uniform, strong transverse seam.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.